The present invention relates to methods of making fibrous sheets in general, and more specifically to a wet-creped process wherein a web is compactively dewatered and thereafter creped, while controlling the permeability of the sheet to facilitate aftercrepe throughdrying and produce products of high bulk.
Methods of making paper tissue, towel, and the like are well known, including various features such as Yankee drying, throughdrying, dry creping, wet creping and so forth. Conventional wet pressing processes have certain advantages over conventional through-air drying processes including: (1) lower energy costs associated with the mechanical removal of water rather than transpiration drying with hot air; and (2) higher production speeds which are more readily achieved with processes which utilize wet pressing to form a web. On the other hand, through-air drying processes have become the method of choice for new capital investment, particularly for the production of soft, bulky, premium quality tissue and towel products.
One method of making throughdried products is disclosed in U.S. Pat. No. 5,607,551 to Farrington, Jr. et al. wherein uncreped, throughdried products are described. According to the ""551 patent, a stream of an aqueous suspension of papermaking fibers is deposited onto a forming fabric and partially dewatered to a consistency of about 10 percent. The wet web is then transferred to a transfer fabric travelling at a slower speed than the forming fabric in order to impart increased stretch into the web. The web is then transferred to a throughdrying fabric where it is dried to a final consistency of about 95 percent or greater.
There is disclosed in U.S. Pat. No. 5,510,002 to Hermans et al. various throughdried, creped products. There is taught in connection with FIG. 2, for example, a throughdried/wet-pressed method of making creped tissue wherein an aqueous suspension of papermaking fibers is deposited onto a forming fabric, dewatered in a press nip between a pair of felts, then wet-strained onto a through-air drying fabric for subsequent through-air drying. The throughdried web is adhered to a Yankee dryer, further dried, and creped to yield the final product.
Throughdried, creped products are also disclosed in the following patents: U.S. Pat. No. 3,994,771 to Morgan, Jr. et al.; U.S. Pat. No. 4,102,737 to Morton; and U.S. Pat. No. 4,529,480 to Trokhan. The processes described in these patents comprise, very generally, forming a web on a foraminous support, thermally pre-drying the web, applying the web to a Yankee dryer with a nip defined, in part, by an impression fabric, and creping the product from the Yankee dryer.
As noted in the above, throughdried products tend to exhibit enhanced bulk and softness; however, thermal dewatering with hot air tends to be energy intensive and requires a relatively permeable substrate. Thus, wet-press operations are preferable from an energy perspective and are more readily applied to furnishes containing recycle fiber which tends to form webs with less permeability than virgin fiber.
The state of the art is further illustrated in the following patents. It will be appreciated that high production rates (sheet speeds) are exceedingly important to the viability of many production processes. In connection with paper manufacture, it has been suggested, for example, to employ an air foil to stabilize web transfer off of a Yankee dryer in order to maintain suitable production rates. There is disclosed, for example, in U.S. Pat. No. 5,891,309 to Page et al. a foil positioned adjacent a Yankee dryer above a creping doctor. The foil is designed to stabilize the web as it leaves the dryer and includes an air deflector positioned tangent to the Yankee dryer. The web is held against the bottom side of the foil by one or more Coanda air jets which are directed over the bottom surface of the foil. The jets are intended to prevent the web from sticking to the bottom surface of the foil while creating a Bernoulli effect which holds the web against the foil. See also, U.S. Pat. No. 5,512,139, to Worcester et al. which discloses a static foil (46, FIG. 1) intended to stabilize a sheet. Another method of facilitating transfer off a can dryer is disclosed in U.S. Pat. No. 5,232,555 to Daunais et al.
U.S. Pat. No. 5,851,353 to Fiscus et al. teaches a method for can drying wet webs for tissue products wherein a partially dewatered wet web is restrained between a pair of molding fabrics. The restrained wet web is processed over a plurality of can dryers, for example, from a consistency of about 40 percent to a consistency of at least about 70 percent. The sheet molding fabrics protect the web from direct contact with the can dryers and impart an impression on the web.
U.S. Pat. No. 5,087,324 to Awofeso et al. discloses a delaminated stratified paper towel. The towel includes a dense first layer of chemical fiber blend and a second layer of a bulky anfractuous fiber blend unitary with the first layer. The first and second layers enhance the rate of absorption and water holding capacity of the paper towel. The method of forming a delaminated stratified web of paper towel material includes supplying a first furnish directly to a wire and supplying a second furnish of a bulky anfractuous fiber blend directly onto the first furnish disposed on the wire. Thereafter, a web of paper towel is creped and embossed.
U.S. Pat. No. 5,494,554 to Edwards et al. illustrates the formation of wet press tissue webs used for facial tissue, bath tissue, paper towels, or the like, produced by forming the wet tissue in layers in which the second formed layer has a consistency which is significantly less than the consistency of the first formed layer. The resulting improvement in web formation enables uniform debonding during dry creping which, in turn, provides a significant improvement in softness and a reduction in linting. Wet pressed tissues made with the process according to the ""554 patent are internally debonded as measured by a high void volume index.
Other processes such as wet crepe, throughdry processes have been suggested in the art and practiced commercially. One such process is described in U.S. Pat. No. 3,432,936 to Cole et al. The process disclosed in the ""936 patent includes: forming a nascent web on a forming fabric; wet pressing the web; drying the web on a Yankee dryer; creping the web off of the Yankee dryer; and through-air drying the product.
Another wet crepe, through-air dry process is suggested in U.S. Pat. No. 4,356,059 to Hostetler. In the ""059 patent there is disclosed a process including: forming a nascent web on a forming fabric; drying the web on a can dryer; creping the web off of the can dryer; through-air drying the web; applying the dry web to a Yankee dryer; creping the web from the Yankee dryer and calendering the product.
Wet crepe, through-air dry processes have not met with substantial commercial success since the process rates, product quality and machine productivity simply could not meet the demanding criteria required in the industry.
It has been found in accordance with the present invention that a wet crepe process can run at high productivity and provide a range of quality products provided certain elements of the process are properly controlled. Salient features of the present invention include: (a) creping a partially dried web off a heated dryer and (b) controlling the microstructure of the wet web such that the web is suitable for transpiration or throughdrying at high rates. These features and numerous other aspects of the present invention are described in detail below.
It has been found in accordance with the present invention that fibrous sheets are advantageously produced from a furnish of fibers by preparing a nascent web, controlling its porosity and microstructure while compactively dewatering the web, and at least partially throughdrying the web wherein airflow through the sheet exhibits a dimensionless characteristic Reynolds Number of less than about 1 and a characteristic dimensionless throughdrying coefficient of from about 4 to about 10. In this airflow regime, viscous pressure drop through the sheet is significant. A particularly preferred process involves: (a) depositing an aqueous furnish onto a foraminous support; (b) compactively dewatering the furnish to form a web; (c) applying the dewatered web to a heated rotating cylinder and drying the web to a consistency of greater than about 30 percent and less than about 90 percent; (d) creping the web from the heated cylinder at the aforesaid consistency; and (e) throughdrying the web subsequent to creping it from the cylinder to form the absorbent sheet. The furnish composition and the processing of steps (a), (b) and (c) as well as the creping geometry, the moisture profile of the web upon creping, the web adherence to the heated cylinder and the throughdrying conditions are controlled such that airflow through the sheet exhibits a characteristic Reynolds Number of less than about 1 and a characteristic throughdrying coefficient of from about 4 to about 10. In a typical embodiment, a method of making absorbent sheet includes: (a) depositing an aqueous cellulosic furnish on a foraminous support to form a nascent web; (b) compactively dewatering the web in a transfer nip while transferring the web to a Yankee cylinder; (c) drying the web to a consistency of from about 30 to about 90 percent on the Yankee cylinder; (d) creping the web from the Yankee cylinder; (e) transferring the web over an open draw to a throughdrying fabric while aerodynamically supporting the web; (f) re-wetting the web with an aqueous composition; (g) wet molding the re-wet web on the throughdrying fabric; and (h) throughdrying the re-wet web to form an absorbent sheet wherein airflow through the sheet exhibits a characteristic Reynolds Number of less than about 1 and a characteristic dimensionless throughdrying coefficient of from about 4 to about 10.
The novel products of the invention include fibrous sheet such as absorbent cellulosic sheet having a void volume fraction of from about 0.55 to about 0.85, a wet springback ratio of at least about 0.6 and a hydraulic diameter of from about 3xc3x9710xe2x88x926 ft to about 8xc3x9710xe2x88x925 ft. The products are distinguished from conventional wet-pressed products by their wet resilience and are distinguished from conventional throughdried products by virtue of their hydraulic properties. Conventional throughdried products are generally characterized by void volume fractions of greater than about 0.72 and hydraulic diameters of greater than about 8xc3x9710xe2x88x926 ft. The products of the present invention typically have a hydraulic diameter of less than about 7xc3x9710xe2x88x926 ft when the void volume fraction exceeds about 0.8 or so. Novel products of the present invention in some embodiments exhibit relatively high wet springback ratios as well as high internal bond strength. In general, such products exhibit a wet springback ratio of from about 0.4 to about 0.8 as well as an internal bond strength parameter of greater than about 140 g/in/mil.
There is provided in yet another aspect of the present invention a process for making fibrous sheet wherein the process generally includes depositing an aqueous furnish onto a foraminous support, compactively dewatering the furnish to form a web, applying the web to a heated rotating cylinder where the web is dried to a consistency of greater than about 30 percent and less than about 90 percent, creping the web from the heated cylinder at the aforesaid consistency and throughdrying the creped web; the improvement being controlling the characteristic void volume of the as-creped creped web such that said web exhibits a characteristic void volume upon creping in grams/g of greater than about 9.2-0.048X wherein X is the GMT of the as-creped product (grams/3xe2x80x3) divided by the basis weight of the as-creped product (lbs/3000 ft2).
In a further aspect of the present invention, there is provided a wet-crepe, throughdry process for making fibrous sheet, including the steps of: (a) depositing an aqueous furnish onto a foraminous support; (b) compactively dewatering the furnish to form a cellulosic web; (c) applying the dewatered web to a heated rotating cylinder and drying the web to a consistency of greater than about 30 percent and less than about 90 percent; (d) creping the web from the heated rotating cylinder at the aforesaid consistency of greater than about 30 percent and less than about 90 percent, wherein the furnish composition and processing of steps (a), (b) and (c), as well as the creping geometry, the temperature profile of the web upon creping, the moisture profile of the web upon creping and the web adherence to the heated cylinder are controlled such that the characteristic void volume of the web in grams/g upon creping is greater than about 9.2-0.048X wherein X is the GMT of the as-creped product (grams/3xe2x80x3) divided by the basis weight of the as-creped product (lbs/3000 ft2); and (e) throughdrying the web subsequent to creping said web from said heated cylinder to form said sheet.
The void volume of the final products is also characteristic of various processes of the invention. Thus a wet crepe, throughdry process for making fibrous sheet may include the steps of: (a) depositing an aqueous furnish onto a foraminous support; (b) compactively dewatering the furnish to form a web; (c) applying the dewatered web to a heated rotating cylinder and drying the web to a consistency of greater than about 30 percent and less than about 90 percent; and (d) creping the web from the heated cylinder at the consistency of greater than about 30 percent and less than about 90 percent, wherein the furnish composition and processing of steps (a), (b) and (c), as well as the creping geometry, temperature profile of the web upon creping, moisture profile of the web upon creping and web adherence to the heated rotated cylinder are controlled; and (e) throughdrying the web subsequent to creping the web from the heated cylinder to form the sheet, wherein the void volume of the sheet in grams/g is greater than about 9.2-0.048X wherein X is the GMT of the product (grams/3xe2x80x3) divided by the basis weight of the product (lbs/3000 ft2).
In some embodiments of the present invention there is provided a method of making absorbent sheet including delamination creping including the steps of: (a) depositing an aqueous furnish onto a foraminous support; (b) compactively dewatering the furnish to form a web; (c) applying the web to a heated rotating cylinder; (d) maintaining the surface of the rotating cylinder at an elevated temperature relative to its surroundings so as to produce a temperature gradient between the air and cylinder side of the web; (e) drying the web on the cylinder to a consistency of between about 30 and about 90 percent; (f) creping said web from said cylinder, wherein said creping is operative to delaminate said web and said web exhibits a characteristic void volume upon creping in grams/g of greater than about 9.2-0.048X wherein X is the GMT of the as-creped product (grams/3xe2x80x3) divided by the basis weight of the as-creped product (lbs/3000 ft2); and (g) throughdrying the web to form the sheet. The delamination process noted above may also be defined in terms of the product produced thereby or in other words, an inventive method likewise includes: (a) depositing an aqueous furnish onto a foraminous support; (b) compactively dewatering the furnish to form a web; (c) applying the web to a heated rotating cylinder; (d) maintaining the surface of the rotating cylinder at an elevated temperature relative to its surroundings so as to produce a temperature gradient between the air and cylinder sides of the web; (e) drying the web on the cylinder to a consistency of between about 30 to about 90 percent; (f) creping the web from the cylinder, wherein the creping is operative to delaminate the web; and (g) drying the web to form the absorbent sheet, wherein the void volume in grams/g of the sheet is greater than about 9.2-0.048X wherein X is the GMT of the sheet (grams/3xe2x80x3) divided by the basis weight of the sheet (lbs/3000 ft2). Delamination of a sheet refers to the fact that a creped sheet has a reduced density about its center, that is, a reduced fiber density in the interior of the sheet. In the extreme, the product is separated into separate plies and the fiber density approaches 0 at a plane in the interior of the product. Further aspects and advantages of the present invention are described in detail hereinafter.
As used herein, terminology is given its ordinary meaning unless otherwise defined or the definition of the term is clear from the context. For example, the term percent or % refers to weight percent and the term consistency refers to weight percent of fiber based on dry product unless the context indicates otherwise. Likewise, xe2x80x9cppmxe2x80x9d refers to parts by million by weight, and the term xe2x80x9cabsorbent sheetxe2x80x9d refers to tissue or towel made from cellulosic fiber.
The terms xe2x80x9cfibrousxe2x80x9d, xe2x80x9caqueous furnishxe2x80x9d and the like include all sheet-forming furnishes and fibers. The term xe2x80x9ccellulosicxe2x80x9d is meant to include any material having cellulose as a major constituent, and, specifically, comprising at least 50 percent by weight cellulose or a cellulose derivative. Thus, the term includes cotton, typical wood pulps, cellulose acetate, cellulose triacetate, rayon, thermomechanical wood pulp, chemical wood pulp, debonded chemical wood pulp, mikweed, and the like. xe2x80x9cPapermaking fibersxe2x80x9d include all known virgin or recycle cellulosic fibers or fiber mixes comprising cellulosic fibers. Fibers suitable for making the webs of this invention comprise any natural or synthetic cellulosic fibers including, but not limited to: nonwood fibers, such as cotton fibers or cotton derivatives, abaca, kenaf, sabai grass, flax, esparto grass, straw, jute hemp, bagasse, milkweed floss fibers, and pineapple leaf fibers; and wood fibers such as those obtained from deciduous and coniferous trees, including softwood fibers, such as northern and southern softwood kraft fibers; hardwood fibers, such as eucalyptus, maple, birch, aspen, or the like. Woody fibers may be prepared in high-yield or low-yield forms and may be pulped in any known method, including kraft, sulfite, groundwood, thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP) and bleached chemithermomechanical pulp (BCTMP). High brightness pulps, including chemically bleached pulps, are especially preferred for tissue making, but unbleached or semi-bleached pulps may also be used. Recycled fibers are included within the scope of the present invention. Any known pulping and bleaching methods may be used. Synthetic cellulose fiber types include rayon in all its varieties and other fibers derived from viscose or chemically modified cellulose. Chemically treated natural cellulosic fibers may be used such as mercerized pulps, chemically stiffened or crosslinked fibers, sulfonated fibers, and the like. Suitable papermaking fibers may also include recycled fibers, virgin fibers, or mixtures thereof.
Unless otherwise indicated, xe2x80x9cgeometric mean tensile strengthxe2x80x9d (GMT) is the square root of the product of the machine direction tensile strength and the cross-machine direction tensile strength of the web. Tensile strengths are measured with standard Instron test devices which may be configured in various ways, one of which may be described as having a 5-inch jaw span or more using 3-inch wide strips of tissue or towel, conditioned at 50% relative humidity and 72xc2x0 F. for at least 24 hours, with the tensile test run at a crosshead speed of 1 in/min. As discussed below in connection with the internal bond strength parameter, the 3xe2x80x3 GMT is divided by 3 for convenience in expressing the parameter in g/in/mil.
The xe2x80x9cvoid volumexe2x80x9d, as referred to hereafter, is determined by saturating a sheet with a nonpolar liquid and measuring the amount of liquid absorbed. The volume of liquid absorbed is equivalent to the void volume within the sheet structure. The void volume is expressed as grams of liquid absorbed per gram of fiber in the sheet structure. More specifically, for each single-ply sheet sample to be tested, select 8 sheets and cut out a 1 inch by 1 inch square (1 inch in the machine direction and 1 inch in the cross-machine direction). For multi-ply product samples, each ply is measured as a separate entity. Multiple samples should be separated into individual single plies and 8 sheets from each ply position used for testing. Weigh and record the dry weight of each test specimen to the nearest 0.0001 gram. Place the specimen in a dish containing POROFIL(trademark) liquid, having a specific gravity of 1.875 grams per cubic centimeter, available from Coulter Electronics Ltd., Northwell Drive, Luton, Beds, England; Part No. 9902458.) After 10 seconds, grasp the specimen at the very edge (1-2 millimeters in) of one corner with tweezers and remove from the liquid. Hold the specimen with that corner uppermost and allow excess liquid to drip for 30 seconds. Lightly dab (less than xc2xd second contact) the lower corner of the specimen on #4 filter paper (Whatman Ltd., Maidstone, England) in order to remove any excess of the last partial drop. Immediately weigh the specimen, within 10 seconds, recording the weight to the nearest 0.0001 gram. The void volume for each specimen, expressed as grams of POROFIL per gram of fiber, is calculated as follows:
void volume=[W2xe2x88x92W1)/W1], 
wherein
xe2x80x9cW1xe2x80x9d is the dry weight of the specimen, in grams; and
xe2x80x9cW2xe2x80x9d is the wet weight of the specimen, in grams.
The void volume for all eight individual specimens is determined as described above and the average of the eight specimens is the void volume for the sample.
The dimensionless void volume fraction and/or void volume percent is readily calculated from the void volume in grams/gm by calculating the relative volumes of fluid and fiber determined by the foregoing procedure, i.e., the void volume fraction is the volume of Porofil(copyright) liquid absorbed by the sheet divided by the volume of fibrous material plus the volume of Porofil liquid absorbed (total volume) or in equation form
xe2x80x83void volume fraction=(void volumexc3x97specific volume of fluid)/(void volumexc3x97specific volume of fluid+specific volume of fiber)=void volumexc3x970.533/(void volumexc3x970.533+specific volume of fiber)
Unless otherwise indicated, the specific volume of fiber is taken as unity. Thus a product having a void volume of 6 grams/gm has a void volume fraction of 3.2/4.2 or 0.76 and a void volume in percent of 76% as that terminology is used herein.
The products and processes of the present invention are advantageously practiced with cellulosic fiber as the predominant constituent fiber in the furnishes and products, generally greater than 75% by weight and typically greater than 90% by weight of the product. Nevertheless, as one of skill in the art will appreciate, the invention may be practiced with other suitable furnishes.